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New class of rhizobia-forming bacteria identified

A second class of the Proteobacteria can form rhizobia, suggesting this ability is more widespread than previously thought.

David Bruce(david.bruce@biomedcentral.com)

The Leguminosae is recognised as the largest plant family with approximately 18,000 documented species. Their success is in no small part due to their ability to form nitrogen-fixing symbioses with rhizobial bacteria. All rhizobia so far identified belong to the α-subclass of the Proteobacteria. In June 21 Nature, Lionel Moulin and colleagues at l'Institut de Recherché pour le Dévelopment, Paris report the discovery of legume nodulation by members of the ß-subclass of the Proteobacteria.

Moulin et al. characterized rhizobia from a number of tropical legumes and found that those from the nodules of Aspalatus and Machaerium were phylogenetically distant from known rhizobia (Nature 2001, 411:948-949). Analysis of strain STM678 from the South African legume A. carnosa showed it to be most closely related to the Burkholderia bacteria (~97% identity), a genus within the ß-subclass of the Proteobacteria. A subsequent PCR screen of this strain revealed the presence...

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